The electrolysis of aqueous solutions of ionizable chemical compounds, particularly of brine solutions, in a cell equipped with an anode and a cathode separated by a porous diaphragm, barrier, or septum, is well known in the art. In most instances, such cells, be they two compartment or multicompartment cells, are operated under conditions such that ionic and molecular migration through the porous diaphragm occurs to a substantial degree resulting in contamination of the cathode liquor with undecomposed electrolyte and of the anode liquor with reaction products of the cathodic material and the anodic materials.
It has been proposed to replace the porous diaphragm in such cells with a barrier impervious, or substantially so, to both liquids and gases thereby to control both ionic and molecular migration during electrolysis. Many patents, such as U.S. Pat. Nos. 2,967,807, 3,390,055, and French Pat. No. 1,510,265, disclose electrolytic cells incorporating as the barrier, membranes fabricated from synthetic organic ion-exchange resins. However, such resins have not been entirely satisfactory due to their sensitivity to the strong caustic or acidic media with which they were in contact during use, or the voltage drop through the membrane became excessive as the caustic concentration increased, or such resins and/or their fabrication were too costly.
More recently, as disclosed in copending application Ser. No. 212,171 of E. H. Cook Jr. et al., filed Dec. 27, 1971, high purity products in high yield without undue loss of electrical current and loss of product yield due to ionic and/or molecular migration could be obtained when the electrolytic cells comprised a barrier formed from a permselective membrane composed essentially of a hydrolyzed copolymer of tetrafluoroethylene and a sulfonated perfluorovinyl ether. This barrier material was found to possess the desirable attributes of retaining its effectiveness, that is chemical inertness, over extended periods of use in electrolytic cells. However, the barrier material was found to expand during use under the cell operating conditions. This resulted in the development of folds and/or creases on the anode side of the barrier and the entrapment of chlorine gas (when brine was used as the anolyte) in the folds and/or creases. Such entrapment of gaseous products resulted in half cell voltage readings in both anode and cathode compartments which were about 0.4 to 0.6 volts higher than normally expected.
Careful voltage analysis of the operating cells has shown that the erratic voltages were, in part, associated with a gasing problem at the anode. It has been observed that at times gas would accumulate at the upper portion of the anode structure causing an uneven distribution of the electrical current on the anode.
It is postulated that membranes that have been installed without pretreatment become hydrolyzed during operation of an electrolytic cell and upon taking up water they expand. The expansion allows a sagging or blistering effect to take place and the blisters or sags fill up with trapped chlorine gas. The trapped gas causes extra resistance between the anode and the cathode, or buffer compartment and causes the anode half cell potential to increase due to increased current density through the rest of the membrane.